Design1stCode2nd

OK, showing my ignorance, the whistliing noise is the turbo I assume?

Ike

It's pressurized air from a blow of valve (BoV), connected to the intercooler tubing, that leads to the turbo venting air to the atmosphere.

Ike

There are some classic threads on Nasioc, I never posted much there since anything I ever wanted to know was just a search away and it was such a large well established community that I felt like the new kid in school.

ASH8

I just thought the same think Zoom.....are you OK Ike.....I ,we will miss your SUB ing..

Ike

Here's a pic for you for old times sake then. A few mods ago but it'll do

Design1stCode2nd

So would a Stock Evo sound like this or is it due to the BoV being added? There are a couple of WRXs in my neighborhood that sound like this.

Ike

It's due to the BoV being added. Also those guys in your neighborhood have no idea how to mod a car since BoV that's vent mess with cars still equiped with a MAF.

Steiner

No any WRX, STi or Evo with the "pssshhhhhhhhtttttt" sound installed an aftermarket BOV.

The bad thing about those loud blow-off valves is they can be really detrimental tp daily driveability and even performance on certain cars. The ECU (engine control unit) in the WRX, Evo and STi all use what's called a MAF (mass air flow) sensor. Without going into too much detail and at the risk of regurgitating technical info I don't fully understand myself, the car's ECU basically measures airflow and airflow properties at a few points between the throttle body and the airbox. The ECU compensates accordingly, and in real time, as airflow dynamics change. It's my understanding that there are dozens of different variables the ECU is monitoring. However there are a few variables it cannot account for. On the Evo one of those is loss in total air volume. When you let off the throttle under boost, the stock BOV (blow off valve) on the Evo's turbo recirculates the unused compressed air back into the intake tract. In this fashion none of the air between the airbox and the throttle body is ever lost...ever. It's a constant that the ECU relies on to effectively manage things. However with an aftermarket VTA (vent to atmosphere) style BOV the unused compressed air is released to the atmosphere for that distinctive "pshhhhhhhhhhtttt" sound. The Evo's ECU cannot account for the loss in air volume that the MAF sensor is recording so it will do some goofy things to offset what it perceives as an engine problem. One of those things is to increase fuel flow in order to maintain its target AFR (air-to-fuel ratio). The car can then run unneccesarily rich, idle poorly and gas mileage can suffer. In a similair way aftermarket intakes can occasionally cause problems on a MAF based ECU as well because they introduce more air volume than the ECU needs to maintain its target AFR...although I've heard with the proper ECU reflash some tuners claim they can offset this variable to gain a quicker spool and a little power. Seems like for every one tuner that recommends an aftermarket intake for the Evo there are two that pop up to oppose it though.

I hope this explained it a little better. I'm sure I left out a ton of details, but that's at least my basic understanding of a MAF based ECU and why a VTA BOV can be more trouble than it's worth.

Design1stCode2nd

Actually very helpful, Thanks. I was doing some work in the yard and heard them go by, figured it was some sort of mod or a turbo since I've never had a car with a a turbo on it. Didn't sound good to me, I'd prefer just a good sounding engine myself.